1. Field of the Invention
The present invention relates to an antenna device incorporated in a small-sized radio terminal, especially to an antenna device showing a good reception characteristic for high-frequency radio signals regardless of direction and a portable radio terminal provided with the antenna.
2. Description of the Prior Art
In recent years, a portable radio terminal typified by a mobile phone often uses radio signals in a high-frequency band. For instance, the frequency used in the third generation mobile phone goes beyond 2 GHz (gigahertz), and has a tendency to shift to the higher frequencies.
While a portable radio terminal is getting smaller and smaller, it has limitations for further miniaturization thereof from the operational point of view. Accordingly, as the higher frequencies are used, the length of a housing of a portable radio terminal often becomes beyond more than half of the wavelength λ, being λ/2.
As the length of the housing becomes longer with respect to the frequency, the radiation characteristic changes because of the current of the housing. Therefore, as represented by a half wave dipole antenna, it is not possible to have the uniform field emission pattern characteristic in a horizontal surface, and an abrupt drop (null point) appears in the horizontal surface.
In the case where the frequency is around 1 GHz, the wavelength is about 30 centimeter. Accordingly, when the electrical length including that of an antenna is equal to or less than λ/2, that is, when the length of the housing is equal to or less than 7.5 centimeter, and the electrical length of the antenna is equal to or less than λ/4, its field emission pattern characteristic becomes similar to that of the half wave dipole antenna. Thus, it is possible to have a relatively uniform field emission pattern characteristic in the horizontal surface.
However, in the case where the frequency is around 2 GHz, the wavelength is about 15 centimeter. Accordingly, even when the length of the housing of a radio terminal is around 10 centimeter, the electrical length including that of an antenna is about the same as or more than the wavelength. Thus, the field emission pattern characteristic does not become uniform because of the current of the housing, and an abrupt drop (null point) arises in the horizontal surface.
There has been the problem that it is not possible for a portable radio terminal to stably receive radio signals if the housing thereof has such a field emission characteristic and the reception characteristic changes depending on the direction of the terminal.
As a prior art document concerning an antenna of a portable radio terminal, Japanese utility model patent application laid-open No. 62-161410 discloses an antenna for a radio terminal, in which a platy radiating element is disposed parallel to the surface of a metallic housing of a radio terminal, one end of the platy radiating element is connected to the housing of the radio terminal and immobilized, a feeder cable is connected to a designated position on the platy radiating element, and a rod-shaped parasitic element is placed on the housing of the radio terminal.
This prior art realizes an antenna of a small size and also broadband characteristic by resonating the reverse F-shaped antenna of the platy radiating element and the rod-shaped parasitic element with each different resonance frequency.
Further, Japanese patent application laid-open No. 2004-56319 discloses a null-less antenna intended to fill in a null.
In the first prior art, while the radio terminal shows a good radiation characteristic in two frequency bands, the generation of a null has not been conceived. Therefore, when the art is applied to an antenna in a portable radio terminal, radio signals cannot be transmitted and received between a radio terminal and another radio terminal or a base station depending on the aspect of the radio terminal.
Also, the second prior art is so-called antenna array having a plurality of antennas, and therefore, it is difficult to apply the art to a portable radio terminal being required to be made smaller and lighter.
As above, it has not been provided an antenna device capable of being applied to a portable radio terminal and also showing a good radiation characteristic regardless of the direction of the terminal.